In modern aircraft there are usually a fairly large number of devices that have to be provided with warm and pressurized air. One of the most important consumers of this kind is the air conditioning system of a commercial aircraft, which due to the great flight altitude of modern commercial aircraft and the low outside pressure prevailing there and the low outside temperatures has to create artificially an interior atmosphere that is tolerable for passengers. In order to be able to supply air at a high temperature for such air-consuming devices, a portion of the pneumatic air, also referred to as bleed air, is generally bled off from the aircraft engines at certain positions.
This air is often air which derives from one of the compression stages of the engine and is therefore under great pressure (up to approximately 50 PSI, corresponding to approximately 3.5 bar) and can have a high temperature of up to approx. 400° C. This bleed air must then be transported from the engines to the devices of the aircraft, which generally takes place via a pipework system.
It is normally advisable to cool the air from the engine to approx. 200-260° C. by means of a temperature control system (EBAS=“Engine Bleed Air System”) before it is supplied to the consuming devices. This can be achieved e.g. by interaction with very cold air from the aircraft environment in a heat exchanger. The EBAS generally has an electronic temperature control system, which registers the temperature of the cooled air and controls it as required. This air can then be routed to the consumers via pipes consisting mostly of titanium alloys.
If the pipework system has damaged points, the very hot bleed air, which is under high pressure, can escape at these and act on the surroundings of the pipework system. The heating associated with this can cause damage to aircraft components that come into contact with the hot air.
In particular, power lines, fuel lines, hydraulic lines or other sensitive parts close to the pipework system can be affected by damage in this case. Even bearing members of an airframe, for example, can be damaged. Such damage can possibly seriously impair the flight safety of an aircraft and entail grave consequences for the safety of the passengers and crew up to the possible crash of the aircraft.
For this reason sensors for detecting fractures are now fitted in aircraft along the entire pipework system, these being evaluated by a leakage monitoring system that is also known as OHDS (“OverHeat Detection System”). The sensors are normally surface sensors, which consist of cylindrical wires a few millimeters thick that contain between the core and sheath a filling that has a temperature-dependent electrical resistance. Below a certain response temperature, which can be set within certain limits during production, the resistance is very great. However, if this response temperature is exceeded, the resistance is abruptly reduced by several orders of magnitude. Such a change in resistance can easily be detected electronically by a monitoring device.
If hot air emerges from the pipework system through a leak in such a system, it heats the surrounding sensors until these reach the response temperature and the monitoring system detects the leak with reference to the change in resistance. Additional electronics in the monitoring system (OHDS) then interrupt the air supply in the section concerned by closing an assigned shutoff valve, which is closed in the de-energized state, by turning off the valve power supply.
The temperature control system EBAS and the leakage monitoring system OHDS are generally realized with the same hardware in a common computer BMC (“Bleed Monitoring Computer”).
EP 0 175 698 B1 discloses a bleed air supply system with a leakage monitoring device with a control device, which is connected to valves for turning off the bleed air flow.
In DE 10 2004 039 667 A1, an air supply device is described in which an air supply can be sealed off via a valve that can be controlled on the basis of signals from temperature sensors.
In the system according to the prior art described above, if the temperature control system fails, the temperature of the air in the pipework system can rise to the temperature of the uncooled bleed air, thus up to approx. 400° C. The pipework system lying downstream of the EBAS is not designed for such hot air, and in particular seals joining individual pipes can be strongly affected and degenerate if they are exposed to such hot air. A leak with hot air flowing out can arise due to this.
Furthermore, if also a system fault in the monitoring computer BMC occurs, this would affect the leakage monitoring device, which is in itself independent of the temperature control system and in such a case (EBAS failure and fault in BMC) then would not automatically shut off the bleed air flow, with possible serious consequences for flight safety.
These possible serious consequences must be taken into consideration in development in that a high safety level (Development Assurance Level, DAL) is specified for the bleed air supply system. This leads to a very cost-intensive development process.